1. Field of the Invention
The present invention relates to a vertical trench-gate power semiconductor device.
2. Background Art
Photolithography steps are reduced when forming the source of a trench-gate vertical MOSFET (Metal Oxide Semiconductor Field Effect Transistor, hereinbelow referred to as “UMOS”) by using a structure in which a base trench is formed in a region between trench gate electrodes, a base contact layer is formed at the bottom face of the base trench, and a portion of a source electrode is filled into an interior of the base trench (for example, refer to JP-A 2003-318396 (Kokai)).
Not only can photolithography steps be reduced using such a structure, but the base contact layer can be formed at a deeper position. Therefore, the avalanche withstand capability can be increased. Further, the base layer, which has a resistivity higher than that of the base contact layer, can be made narrow, and therefore the ON resistance can be reduced.
However, in such a structure, metal forming the source electrode may not be able to fill the base trench, resulting in voids. Such voids increase the contact resistance between the base contact layer and the source electrode, causing an avalanche current to flow in the base layer. A potential difference thereby occurs due to resistance in the base layer, and a parasitic bipolar transistor readily occurs. In the case where the parasitic bipolar transistor occurs, the avalanche current undesirably concentrates in one location, and the avalanche withstand capability decreases.
Particularly in a product having a low breakdown voltage, the proportion of the channel resistance due to the ON resistance is large, and it is necessary to increase the channel density and reduce the cell pitch to reduce the ON resistance. However, to reduce the cell pitch, it is necessary to reduce the width of the base trench; and it becomes even more difficult to fill the source electrode into the base trench.
On the other hand, forming the base trench shallower to improve the ease of filling the metal makes the base contact layer distal to the point at which avalanche breakdown occurs, and still the avalanche withstand capability decreases.